Burner safety control system with purging



Feb. 19, 1957 R. J. cAsslDY BURNER SAFETY CONTROL SYSTEM WITH PURGINGOriginal Filed Nov. 12, 1949 Gtornegs Unite rates BURNER SAFETY CONTROLSYSTEM WITH PURGING Robert Il. Cassidy, Mansfield, Ohio, assignor toGeneral Motors Corporation, Detroit, Mich., a corporation of Delaware y4 Claims. (Cl. 153--28) The present invention relates generally tocontrol systems and more particularly to a safety control system for uidfuel burners such as are operated intermittently in accordance withheating requirements, and is a division of S. N. 126,817, tiled November12, 1949 issued March 16, 1954, as Patent No. 2,672,188.

The principalrobject of the invention is to provide a simple andeffective control system for fluid fuel burners that incorporates apurging cycle after cessation of fluid fuel flow.

Another object of this invention is to provide a control system for aheating source in which a thermally responsive resistor is employed tocontinue operation of the fan which supplies combustion air for apredetermined time after stopping of the uid fuel feed to purge theheater of combustible matter and obnoxious fumes.

As will be hereinafter described, the basic control system includes athermally responsive resistor as a temperature-sensitive element subjectto heat of combustion gases and a thermally responsive self-heatingresistor as a timing and voltage regulating element with these elementsso associated in the system as to jointly control a relay for the powersupply to the burner.

In the drawing:

- Fig. l is a circuit diagram of the basic control system.

Fig. 2 is a graph showing certain characteristicsy of one of thethermally responsive resistors.

Fig. 3 is a circuit diagram of the complete control system embodying myinvention. y

Referring rst to the showing in Fig. 1, the reference characters A and Bhave been applied to thermally responsive resistor units which willbehereinafter referred to as a stack resistor and a timing resistor,respectively. Each of these units, as employed in the present system, isa resistance unit having a relatively high negative temperaturecoeicient wherein the resistance to current flow through the resistancedecreases materially as the temperature thereof rises. It will beunderstood that these resistor units may be of any suitable type havingresistance material with the desired negative resistance temperaturecharacteristic. Such materials are well known in the art and mayconsist, for instance, of various semi-conducting materials includingoxides of copper, chrome, iron or manganese ,and combinations oftheseoxides which have been heretofore employed for negativeresistance-temperature coelcient resistor-units.

ln the present system the stack resistor A is located in the exhauststack of the fluid fuel burner so as to be primarily responsive to thetemperature of the combustion or exhaust gases and is preferably sodesigned as to have a relatively low thermal inertia to respond readilyto variations in gas temperature. The timing resistor B is of theself-heating type with its temperature primarily responsive to currentflow through the resistance material and suchtemperature may becontrolled by controlling the dissipation of heat Ygenerated within theresistor due to the 12R loss. g

In the wiring diagram of Fig. l a source of current such arent as thegrounded battery 4 is shown as connected by conductor 6 to the fixedcontact 8 of a control switch, such as a conventional thermostat S,having the movable contact 10 connected to one side of a fixed droppingresistance R by conductor 12. The other side of resistance R s connectedby conductor 14 to the timing resistor B which is grounded at 16. Thisside of the resistance R is also connected by a conductor 18 to one endof a control relay coil C having its other end connected by conductor2t) to the stack resistor A which is grounded at 22. It will be notedthat the stack resistor A is in series with the relay coil C and thatthe timing resistor B is in shunt with the resistor A and coil C. Aconductor 24 connects conductor 12 to a contact 26 of a relay switchhaving a contact arm 28 adapted to move upon proper energization ofcontrol relay coil C to engage both contact 26 and a contact 30 which isconnected by conductor 32 to the grounded load L which may consist of afuel pump or valve, ignition coil, fuel air fan or such other elementsas may be used to operate the burner and provide combustion when thereis a demand for heat by the thermostat S.

Before describing the operation of the complete circuit it is believedit would be desirable to explain the functions of the stack resistor Aand timing resistor B. Considering first the stack resistor A it will beseen that since resistor A is in series with the relay coil C, thevoltage indicated at Eb in Fig. l between conductor 18 and ground willbe shared between these elements. The electrical characteristics of thestack resistor A and relay coil C are so chosen that at a given steadyvalue of voltage Eb, when stack resistor A is cold and of highresistance the greater part of this voltage appears across the resistorA so that relay coil C is not energized, but as the stack resistor A isheated to reduce its resistance, a greater part of the voltage Ebappears across and energizes the relay coil C. If, however, the voltageEb should assume values considerably higher than normal, the relay coilC will be energized even though the stack resistor A is cold and it willtherefore be seen that the relay coil C can be controlled by the ambienttemperature of resistor A and/ or by the value of voltage Eb.

The timing resistor B and fixed resistance R are adapted to control andregulate the voltage Eb. The timing resistor B is designed and operatedso that self-heating from current I causes its resistance to vary withtime until a steady-state condition obtains where the current rise islimited by resistance R as shown by the graph in Fig. 2. In the -finalsteady-state condition voltage Eb is substantially independent of thesource voltage indicated at E. in Fig. l and load current variations dueto the selfregulating action of the timing resistor B. As will be notedhereinafter the initial falling-olf portion of the Eb Voltage-timecharacteristic is employed in the present control system so that therelay coil C will be deenergized at a given time after the closure ofthermostat S if normal combustion fails to occur.

Upon closure of the thermostat S current will flow from the battery ithrough the fixed resistance R and then through conductor 14 and timingresistor B to ground 16, and through conductor 1S, relay coil C,conductor Ztl and stack resistor A to ground 22. If the burner has notbeen in operation for some time so that both the resistors are cool, therelatively large value of the voltage Eb appearing across the timingresistor B, as noted above, will immediately energize the control relaycoil C to thereby close the relay switch and supply current throughconductors 24 and 32 to the loadA L to operate the burner.`

if proper combustion occurs, the stack resistor A will be rapidly heatedby the exhaust gases to thereby reduce its resistance to a very lowvalue and throw a large portion of voltage Eb across the magnet coil C;t Meanwhile the timing resistor B is heating due to its own 12R loss tothereby decrease its resistance and cause voltage Eb to fall toward apredetermined level such that if the stack resistor A is hot, the relaycoil C will remain energized but if the stack, resistor A is not heatedthe relay coil C will not receive suicient voltage to ,retain the relayswitch in closed position. Therefore if the burner fails to ignite orprovide proper combustion when started, or if during operationcombustion should fail or become so poor as to reduce the temperature ofthe stack resistor A below some predetermined point, the relay switchwill open to discontinue burner operation.

To provide the foregoing operation it will be apparent that theelectrical characteristics of the timing resistor B and fixed resistanceR should be so chosen as to allow sufficient time to elapse beforereducing voltage Eb to such an extent as to drop out the relay switch toaccommodate normal ignition of the fuel and heating of the stackresistor A by the burner exhaust gases. 1t is preferable to employ acontrol relay having cut-in and cut-out voltage points close togetherwith a relay coil that requires a minimum of current to reliably operatethe relay switch to avoid any appreciable self-heating of the stackresistor A.

lf burner operation has been discontinued as described above as a resultof initial or subsequent failure of combustion it will be apparent thatthe relay coil C cannot be sufficiently energized to close the relayswitch and resume burner operation as long as the thermostat S is closedsince the timing resistor B remains'heated to maintain voltage Eb at alow value in the previously mentioned steady-state condition. Thereafterthe operation of the burner can only be initiated or attempted if thethermostat S is opened for a short time to permit cooling of the timingresistor B.

Various safety factors of the present control system should be noted inaddition to the normal safety operation described heretofore. lf, forinstance, the stack resistor A should fail by open-circuiting whensubjected to high temperature operation, the relay coil C would bedeenergized, and either open-circuiting or short-circuiting of the relaycoil C would result in opening of the relay switch. If an enclosed typeof mounting is employed for the timing resistor B and it should fail bya short-circuit, the voltage Eb would be reduced to zero to deenergizerelay coil C, and the resistor R may be so constructed as to passshort-circuit current without damage to itself or other parts of thesystem.

With reference to the effects of ambient temperature extremes on theoperating efliciency of the present control system it will be seen thatvery low temperatures produce a higher resistance in the timing resistorB and lengthen the timing thereof to thereby provide more time for theburner to heat up the stack resistor A. Since the stack resistor A isresponsive to the relatively high temperature of the exhaust gases itcan readily be designed so that ambient temperature changes have nomaterial effect and the relay coil C can of course be temperaturecompensated in any known manner, if desired.

Fig. 3 shows the complete control system for use where the electricalloads are heavy and to provide continued operation of the fan whichsupplies combustion air to the burner for a predetermined time after thefuel feed has been stopped to purge the heater of combustible matter andobnoxious fumes.

In the system of Fig. 3, the several parts which correspond to similarparts in Fig. l have been designated by the same reference characters.lt will be noted that what might be termed the basic control system ofFig. l wherein the stack resistor A, timing resistor B, and controlrelay coil C are so related and associated in the circuit as to functionin the manner heretofore described, is incorporated in the system ofFig. 3 without change except for the addition of the resistance Ra inconductor 14 to reduce the voltage across the timing resistor B and theaddition of the variable resistance V in conductor 20 4 to provide foradjustment of the voltage across the stack resistor A. It will also benoted, however, that the control relay coil C does not operate a switchin the load circuit as in Fig. l but controls the energization of apower relay as will be hereinafter described.

As shown in Fig. 3 the conductor 12 is connected by a conductor 36 tothe Contact arm 38 which is adapted to be moved upon energization ofcontrol relay coil C into engagement with a contact 40 connected byconductor 42 to the grounded power relay coil P. When the relay coil Cis not energized, the contact arm 38 is adapted to cngage a contact 44connected by conductor 46 to a grounded warning light or other signal W.The conductor 12 is also connected by a conductor 48 to a contact 50adapted to be engaged by a movable contact arm 52 when the power relaycoil P is energized to supply current through a conductor 54 to operatesuch elements as the fuel pump and high tension ignition system for theburner and any desired elements of the heating system associated withthe burner such as a water circulating pump, warm air blower, etc.

To control the operation of the fuel air fan which is adapted to supplyair to the burner to be mixed with the fluid fuel, the battery 4 orconductor 6 is connected by a conductor 56 to a contact 58 adapted to beengaged by a contact arm 60 movable with contact arm 52 when the powerrelay coil P is energized with the arm 60 connected by conductor 62 tothe fuel air fan or motor operating the same. The operation of the fuelair fan is also controlled in the manner to be hereinafter described bya fan relay coil F, a resistance Rb and a thermally responsive resistorH. A conductor 64 connects conductor 56 to a contact 66 adapted to beengaged by a movable contact arm 68 when the fan relay coil F isenergized. The contact arm 68 is connected to a conductor 70 whichconnects conductor 62 to one end of the relay coil F with the other endof this coil connected by a conductor 72 to the grounded resistance Rband by a conductor 74 to a thermally responsive resistor H which isconnected by conductor 76 to a contact 78 adapted to be engaged by themovable contact arm 60 when the power relay coil P is not energized. Thethermally responsive resistor H is generally similar to the previouslydescribed timing resistor B in that it consists of a resistance unithaving a negative temperature coefficient and is of the selfheating typein which the resistance material is primarily heated by current flowtherethrough to decrease its rcsistance.

In the system of Fig. 3 it will be apparent that the safety controloperation provided by the basic control System which includes the stackresistor A, timing resistor B and control relay coil C is the same asthat of Fig. l with the previously noted exception that the controlrelay coil C does not directly operate a switch in the load circuit butcontrols the energization of the power re lay coil P to operate switcharms 52 and 60 to supply current to the burner, etc. With referencethereto it should be noted that a warning signal W has been provided inFig. 3 which will be energized upon engagement of the switch arm 38 withcontact 44 to call attention to the failure of proper combustionwhenever the thermostat S is closed and calling for heat but the controlrelay coil C has been deenergized in the manner heretofore de scribedupon absence of proper combustion.

When the thermostat Sfcloses upon a demand for heat, the initialoperation of the fuel `air fan is controlled by the energization of thepower relay coil P to move contact arm 60 into engagement with contact58 to thereby connect conductor 56 to conductor 62. As soon as con tactarm 60 engages contact 58 current passes through conductor 70, fan relaycoil F and grounded resistance Rb and energizes the coil F to actuatecontact arm `68 into engagement with contact 66. The closure of thisswitch completes a holding circuit for the fan relay coil F fromconductor 56 through conductor 64 and the switch, and

also connects conductor 56 to conductor 62 through conductors 64 and 70to thereby connect the source of current directly to the fuel air fanand bypass both the thermostat S and the power relay switch contact 58and arm 60.

Whenever either the thermostat S opens or there is a failure of propercombustion, the power relay coil P will be deenergized and the contactarms 52 and 60 will move out of engagement with'the respective contacts50 and 58. The current supply to operate the fuel pump and ignition forthe burner will be thereby interrupted but current supply to operate thefuel air fan will be maintained for predetermined time thereafter by thebypass connection around thermostat S and the power relay contact arm 60and contact 58 through the fan relay switch, as described above. As thecontact arm 60 moves away from contact 58 it engages the contact 78 tocomplete a shunt circuit through conductor 76, resistor H and conductor74 and shunt the resistor H across the fan relay coil F. The resistor His then heated by its resistance to current flow to thereby decrease itsresistance until it has caused the voltage across the relay coil F toreach such a low value that this coil is deenergized or insufficientlyenergized to retain the contact arm 68 in engagement with contact 66 andthe circuit will then be opened by this fan relay switch to disconnectthe fuel air fan, fan relay coil F and resistor H from the source ofcurrent.

Although the preferred basic safety control system and one elaborationthereof have been shown and described herein, it should be understoodthat the present invention is not limited specifically thereto sincemodifications in the basic system and further elaborations thereof willoccur to those skilled in the art and are contemplated as within thescope of the present invention as defined in the claims appended hereto.

I claim:

1. In `a control system for fuel burners, a grounded source of power, apower relay connected thereto to control burner operation including apower switch to control operation of a fan to supply air for combustion,a bypass circuit connected to said source and around said power switchincluding a normally open fan relay switch, an electrically operablemagnetic field creating means connected to said bypass circuit on theside of the power switch controlling the fan remote from the source ofpower and to ground to actuate said fan relay switch upon closure ofsaid power switch, and means connected to said magnetic field creatingmeans to gradually progressively decrease the current flow through themagnetic tield creating means with the opening of the power switch to apoint where the magnetic field creating means releases the fan relayswitch to delay opening of said fan relay switch for a predeterminedtime after Opening of said power switch.

2. The control system set forth in claim l in which said means to delayopening of said fan relay switch includes a resistor connected to saidield creating means whose resistance varies with its temperature inresponse to self-heating by internal resistance to current flow, and

3. In a control system for fuel burners, a grounded source of power, apower relay connected thereto to control burner operation including apower switch to control operation of a fan to supply air for combustionand a power relay coil operatively associated with and arranged to closesaid switch when energized, a'bypass circuit connected to said sourceand around said power switch including a normally open fan relay switchwith a fan relay coil operatively associated therewith to close said fanrelay switch when energized, a resistance, said fan relay coil connectedto said bypass circuit on the side of the power switch controlling thefan remote from the source of power and through said resistance toground and being arranged to be energized by closure of said powerswitch, and a thermally responsive resistor connected to said relaycoil, and additional switching means connected to said resistor andbypass circuit to connect said resistor in shunt with said fan relaycoil byv operation of said power switch when said power relay coil isdeenergized, said resistor having a negative temperature coeicient andresponsive to selfheating by internal resistance to lower its resistanceand reduce the voltage across said fan relay coil to deenergize saidcoil and open said fan relay switch within a predetermined time afterdeenergiz'ation of said power relay coil.

4. In a control system for fuel burners, a grounded current source, apower relay including a rst power switch connected thereto to connectthe current source to a fuel pump and ignition operating means for theburner, a second power switch connected to said source to connect thecurrent source to fan operating means to supply combustion air totheburner, and a power relay coil to close both said switches whenenergized, a thermostat connected between the current source and saidpower relay coil to energize the latter upon demand for heat, a normallyopen bypass circuit around said second power switch and thermostat toconnect the current source to the fan operating means independently ofsaid thermostat, circuit closing means in said bypass circuit tocomplete said bypass circuit, electrical operating means for the circuitclosing means connected to the side of said second power switch remotefrom the source of power and to ground and energized upon closure of thesecond power switch, and means connected to said operating means for thecircuit closing means to gradually progressively decrease the currentllow through said operating means additional switching means connectedto said resistor to connect said resistor in shunt with said iieldcreating means upon opening of said power switch.

with the opening of the second power switch to a point of release of thecircuit closing means to delay the opening of said circuit closing meansfor a predetermined time after opening of said thermostat to deenergizethe power relay coil and open saidy rst and second switches.

References cned in the nie of this patent UNITED STATES PATENTS1,516,646 Roseby Nov. 25, 1924 2,304,613 Vroom Dec. 8, 1942 2,440,700Roesche May 4, 1948 2,616,490 Wilson Nov. 4, 1952 2,672,188 Cassidy Mar.16, 1954

